This invention relates to method and apparatus for converting a data signal into an offset quadrature-phase shift keying (OQPSK) signal for transmission, and more particularly to improved method and apparatus for producing a QPSK modulated signal having a substantially constant amplitude and continuous phase.
Factors that must be considered in selecting a modulation method for transmitting digital information in a satellite communication system include transmit power requirements, spectral efficiency, transmission channel non-linearities, and the complexity of hardware required for implementation. In particular, the frequency spectrum associated with the modulation method must fit within certain constraints. If the communication system uses frequency division multiplex techniques with a channel spacing of 1/T, where T is the symbol interval and the reciprocal thereof is equivalent to a full QPSK mainlobe, any energy out of this frequency range causes adjacent channel interference. QPSK sidelobes can be removed through filtering either pre--or post--output amplification. Power requirements and cost considerations normally make it desirable to employ amplifiers that operate at or near saturation. The choice of pre--or--post-amplification filtering is influenced by the following factors:
post-amplification filtering allows the use of saturated amplifiers without distortion and is often used with wideband transmission. For narrow band or frequency agile applications the complexity of the filter is often prohibitive. PA1 pre-amplification filtering allows for simple spectrum shaping, but does simultaneously introduce signal envelope fluctuations. Non-linearities introduced by an amplifier will reduce these envelope fluctuations and may result in spectral spreading by restoration of sidelobes, which can cause adjacent channel interference and increase the bit error rate.
Hence, some type of PSK modulation with reduced spectral sidelobes and without envelope amplitude variations is preferred. QPSK modulation providing a modulated signal having a constant amplitude and continuous phase is a particularly attractive type of modulation that may be used here. Published articles related to PSK and which are incorporated herein by reference, are:
(1) S. Kato, K. Feher, "Cross-Correlated Phase Shift Keying (XPSK) System With Improved Envelope Fluctuation", 1982 IEEE Proceedings on Communications, pages 2E.1.1-2E.1.5.
(2) K. Feher, T. Le-Ngoc, H. P. Van, "New Modulation Techniques for Low Cost Power and Bandwidth Efficient Satellite Earth Stations", IEEE Transactions on Communications, Vol. Com-30, No. 1, Jan. 1982, pages 275-283.
(3) F. deJager, C. Dekker, "Tamed Frequency Modulation, A Novel Method to Achieve Spectrum Economy in Digital Transmission", IEEE Transactions on Communications, Vol. Com-26, No. 5, May 1978, pages 534-542.
(4) F. de Jager, C. Dekker, D. Muilwijk, "System for Data Transmission by Means of An Angle-Modulated Carrier of Constant Amplitude", U.S. Pat. No. 4,229,821, issued Oct. 21, 1980.
(5) D. Muilwijk, "Correlative Phase Shift Keying-a Class of Constant Envelope Modulation Techniques", IEEE Transactions on Communications, Vol. COM 29, March 1981, pages 226-236.
(6) C. Dekker, "The Application of Tamed Frequency Modulation to Digital Transmission Via Radio", 1979 IEEE, NTC 1979, pages 55.3.1-55.3.7.
(7) D. Muilwijk, J. Noordanus, "Digital Phase Modulation Methods Giving a Band-Limited Spectrum for Satellite Communications", American Institute of Aeronautics and Astronautics, 1980, pages 391-398.
In QPSK modulation, the modulator is preferably arranged for generating an angle modulated carrier signal or vector, of substantially constant amplitude and continuous phase in each symbol interval of length T, said phase being determined by a set of rules. In such a QPSK modulation system an input data signal may be broken into in-phase (I) and quadrature-phase (Q) pulse trains receiving odd and even numbered data bits, respectively. These pulse trains are used to establish the amplitude and phase of the signal vector. The magnitude of the signal vector is maintained substantially constant as the phase thereof varies in order to prevent spectrum spreading. The phase of the signal vector is also caused to be continuous for the same reason. In Ref. 1, a cross-correlated phase shift keying modem employs non-linear switching filters in producing the desired signal. The generation of hard-limited quadrature modulated signals and analysis thereof is discussed in Ref. 2. In the tamed frequency modulation scheme in Ref. 3 and the associated U.S. patent (Ref. 4), digital words that dictate the phase change are obtained from a pair of memories in accordance with particular code rules that require a plurality of successive data bits and information from a quadrant counter.
An object of this invention is the provision of an improved PSK modulation method and apparatus for producing angle modulated carrier signals or phasors of relatively constant amplitude and continuous phase.